Submicron particle removal from gas streams

ABSTRACT

Disclosed are methods and systems for removing submicron particles from a gas stream, in particular from urea prilling off-gas, wherein a Venturi ejector is used. A method comprises contacting a gas stream containing submicron particles in a Venturi ejector with an injected high velocity scrubbing liquid to provide a pumping action, wherein the scrubbing liquid has an initial velocity of at least 2 m/s and wherein the ratio of scrubbing liquid and gas flow is between 0.0005 and 0.0015 (m 3 /h)/(m 3 /h). The disclosure also pertains to a prilling tower having a gas stream treatment system comprising a Venturi ejector at the top of the prilling tower, and to a method of modifying an existing prilling tower.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase of PCT applicationPCT/NL2017/050287 having an international filing date of 9 May 2017,which claims benefit of European patent application No. 16168796.7 filed9 May 2016. The contents of the above patent applications areincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention pertains to removal of particles from a gas stream, inparticular to removing submicron urea dust from a gas stream from a ureaprilling tower.

BACKGROUND

Removal of submicron particles from gas streams is often critical forensuring compliance with emission limits for many industrial processes,such as urea finishing. For instance off-gas from urea prilling towerscontains a relatively large amount and/or large fraction of submicronparticles, for instance compared to the off-gas from a urea fluid bedgranulator. Hence, the removal of submicron urea particles isparticularly important in order to meet ever more stringent regulationsand limits on the emission of urea. The removal of ammonia from ureafinishing off-gas is also necessary. Background references relating tothe removal of urea dust from off-gas from a urea finishing sectioninclude WO 2015/002535 and WO 2015/072854.

It may be observed that particle size distribution of urea prillingtower off-gas has a peak between 0.1 μm and 1 μm aerodynamic particlesize, with a cumulative mass of for example about 70 mg/Nm³ provided byparticles <10 μm (about 50 wt. % total particulate matter, PM). Off-gasfrom urea granulation may for example contain about 25 mg/Nm³ ofparticles <10 μm. For compliance with current and future emissionlimits, significant removal of submicron particles, e.g. urea dust, isessential. Available particle capture technologies generally have verylow efficiency for submicron particles or have a large pressure drop.

In urea prilling, urea melt is supplied at the top of a prilling tower,and distributed as droplets. The urea melt droplets solidify as theyfall down while cooling against a large quantity of upward-moving air.Urea prills are withdrawn from the bottom. The fresh cooling air entersthe bottom of the prilling tower. The off-gas comprising urea andammonia leaves the prilling tower near the top.

A prilling tower can for instance have a height of for example 60 m to80 m. Smaller plants may have a free fall path of 50 m or less. Some ofthe largest plants have prilling towers of 125 m height. Emissions canfor example be 0.5 to 2.5 kg urea dust per ton urea prills (35 to 125mg/Nm³) and about 0.5 to 2.7 kg NH₃ per ton (35-245 mg/Nm³). Urea dustemissions of more than 200 mg/Nm³ have been reported for some existingurea prilling towers. An example indicative air flow for a urea prillingtower is 500 000 Nm³/hr. A larger urea prilling tower may for instancehave 900 000 Nm³/hr, with a urea capacity of 75-100 mt/hr (metric tonper hour).

Older prilling towers frequently vent off-gas directly to air withoutany urea or ammonia abatement. The tower construction generally sets amaximum for the weight for the design of any abatement systems installedon top as part of revamping. The off-gas from some prilling towers has alow pressure drop, in particular the off-gas from prilling towersoperating with natural draft. Existing emission abatement technologiestypically would require large blowers and fans to maintain sufficientpressure drop, since generally submicron particle removal requires highpressure drop. Current systems are hence not suitable for installationon top of an existing prilling tower. The possibility of first bringingthe off-gas to lower elevation through a duct would introduce anadditional significant pressure drop. In view of the large airflows thiswould lead to a significant increase of the power consumption. Theconstruction of a duct from the top to the bottom of the urea prillingtower is also challenging and expensive, and introduces the risk ofplugging in the duct between the prilling tower and the emissionabatement system.

For currently available emission abatement technologies, dust scrubbers,especially if combined with an acid washer to reduce ammonia emissions,are generally considered to be feasible only for forced draft prillingtowers where air fans are available, and not for natural draft ureaprilling towers. For instance U.S. Pat. No. 4,424,072 to Lerner shows inFIG. 1 an apparatus comprising vertical urea prilling tower 11 with aplurality of scrubbers 17 provided above the top of the tower. Thispatent teaches that the apparatus includes facilities for injecting astream of air into the lower part of the prilling tower, accomplished byforced draft, induced draft, or combination thereof. As used in the art,induced draft towers use a centrally located fan at the top and forceddraft cooling towers use a fan located near the bottom.

Accordingly, there is a need for more effective emission abatementsystems and methods that can operate with a low pressure drop and caneffectively remove submicron particles from gas streams. More inparticular, there is a need for better urea and ammonia emissionabatement systems and methods for urea prilling towers.

SUMMARY OF THE INVENTION

In order to better address one or more of the above mentioned desires,the invention pertains, in one of the aspects, to a method for removingsubmicron particles from a gas stream, the method comprising: contactinga gas stream containing submicron particles in a Venturi ejector with aninjected high velocity scrubbing liquid to provide a pumping action,wherein the scrubbing liquid has an initial velocity of at least 25 m/sand/or wherein the ratio of scrubbing liquid and gas flow is between0.0005 and 0.0015 (m³/h)/(m³/h).

The invention also pertains, in a further aspect, to a gas treatmentsystem comprising a Venturi ejector. The invention also pertains, in yeta further aspect, to a urea prilling tower, in particular a naturaldraft urea prilling tower, having two Venturi ejector stages at the top.

A further aspect pertains to a method of modifying existing plants, inparticular urea prilling towers, comprising addition of a Venturiejector.

The invention also pertains to a gas stream treatment method comprisingsubsequently: A) providing a gas stream, B) spraying an aqueous solutioninto the gas stream, C) passing the gas stream through a first ejectorVenturi scrubber having a throat, wherein an aqueous scrubbing liquid issprayed into the gas stream in the direction of the throat, D) sprayingan aqueous solution into the gas stream, F) passing the gas streamthrough a second ejector Venturi scrubber having a throat, wherein anaqueous scrubbing liquid is sprayed into the gas stream in the directionof the throat; preferably using in step C and/or step F a Venturiejector with scrubbing liquid having a velocity and/or ratio asdescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of an embodiment of a system of theinvention.

FIG. 2 depicts a block diagram of an embodiment of a system of theinvention.

FIG. 3 shows a typical particle size distribution and cumulative massfor urea dust in off-gas from a prilling tower.

FIG. 4 shows a typical particle size distribution and cumulative massfor urea dust in off-gas from a urea granulator.

DETAILED DESCRIPTION

The present invention provides in an aspect a method for removingsubmicron particles from a gas stream comprises contacting the gasstream containing submicron particles in a Venturi ejector with ascrubbing liquid. The scrubbing liquid is preferably injected, andpreferably injected with high velocity, in particular to provide apumping action. In this way the gas stream to be treated is drawn intothe Venturi ejector. The scrubbing liquid preferably has an initialvelocity of at least 25 m/s, more preferably at least 50 m/s, even morepreferably at least 100 m/s. Such velocities refer for example to thevelocities at a nozzle opening and/or to the mean droplet velocity overe.g. 1 cm from the nozzle. Preferably, the method comprises injectingscrubbing liquid in a Venturi ejector with a hydraulic nozzle, such as ahigh pressure hydraulic nozzle, e.g. with a nozzle pressure of at least15 bar or at least 18 bar, or with a dual fluid (gas and liquid) nozzle,which may have a pressure lower than 15 bar, in order to provide suchpreferred velocities.

Preferably, scrubbing liquid is injected in a Venturi ejector so as togive a mean droplet diameter of less than 300 μm, more preferably lessthan 200 μm. In some embodiments, the method comprises injectingscrubbing liquid in a Venturi ejector through a nozzle configured toprovide droplets with a mean droplet diameter of less than 300 μm,preferably less than 200 μm. Droplet sizes are e.g. Volume MedianDiameter. This droplet size may contribute to efficient scrubbing.Scrubbing liquid droplets of this size can for example be provided byusing the mentioned scrubbing liquid velocities. In particular ahydraulic nozzle with an injection pressure of at least 15 bar can beused, more preferably at least 18 bar or a dual fluid injector whichuses compressed gases, of e.g. 3 to 6 bar, to finely atomize thescrubbing liquid, of e.g. 3 to 6 bar.

Preferably, the ratio of scrubbing liquid and gas flow is between 0.0005and 0.0015 (m³/h)/(m³/h) in at least one Venturi ejector or in eachVenturi ejector stage. An example ratio is a ratio of scrubbing liquidto gas flow of 0.00010 to 0.0050 (m³/h)/(m³/h), although lower or higherratios are also possible. The ratio is for example based on actual m³ ofthe gas stream to be drawn into the Venturi ejector. Optionally, thescrubbing liquid/gas ratio is in the range of 0.5 to 1.5 l/m³, based onactual m³. Using such ratio, preferably in combination with a scrubbingliquid initial velocity as described, advantageously provides for asurprisingly good scrubbing efficiency in particular with relativelysmall equipment size and a small pressure drop.

Preferably, the method comprises using a plurality of Venturi ejectorsin series. Preferably, the residence time of the gas stream between thefirst and the downstream second Venturi ejector is at least 0.1 s, or atleast 0.20 s, more preferably, at least 0.4 s, for example more than 0.8s. This relates to the residence time between two Venturi ejectorswithout other Venturi ejectors between them. Other elements may bepresent between them, such as a sprayer or a mist eliminator. Using sucha residence time between two consecutive Venturi ejectors in seriescontributes to particle growth, especially of submicron particles. Thisimproves particle capture in for example a downstream particle removaldevice such as a mist eliminator.

Preferably, a first upstream and a second downstream Venturi stage areused each comprising one Venturi ejector or a plurality of parallelVenturi ejectors. Preferably, the first scrubbing liquid of the firstVenturi stage comprises at least 10 wt. % or at least 20 wt. % or atleast 30 wt. % or at least 40 wt. % dissolved material, such as 20 to 55wt. % or 40 to 50 wt. %, and/or such amounts of hydrophilic material, orsuch amounts of material of components removed from the gas stream.Preferably, the dissolved material is urea and the scrubbing liquidcomprises such amount of urea. In some embodiments, the first scrubbingliquid comprises less than 90 wt. % water, or less than 80 wt. %, oreven less than 60 wt. %. The scrubbing liquid used in the seconddownstream Venturi stage and/or in a spray nozzle downstream of thefirst Venturi stage preferably comprises 0 wt. % to 5.0 wt. % dissolvedmaterial, more preferably less than 2.0 wt. % dissolved material, inparticular for urea. Preferably, the second scrubbing liquid comprises80 wt. % to 100 wt. % water, or at least 90 wt. %, or at least 95 wt. %water. Preferably, the scrubbing liquid used in the first Venturiejector stage (first scrubbing liquid) has a higher concentration ofdissolved material than the scrubbing liquid used in the second Venturiejector stage (second scrubbing liquid), preferably at least 3 timeshigher or at least 5 times higher or more preferably at least 10 timeshigher. The scrubbing liquid used in the first Venturi stage, inparticular in the most upstream Venturi ejector, is for example thefirst point of contact with the off-gas.

The first scrubbing liquid is generally recirculated to provide suchconcentrations of dissolved material (e.g. urea) so as to allow foreasier disposal, in particular urea recovery. In particular in case ofoff-gas (cooling air flow) from prilling or granulation, the prilled orgranulated material is preferably recovered. This applies in particularto urea granulation or prilling. The recovered urea may be incorporatedin a urea-containing product, e.g. the prills or granules. Generally, astream comprising such concentrations of urea, e.g. 40 wt. % to 50 wt. %is withdrawn from the first Venturi stage and/or an upstream quenchstage as purge and/or blowdown stream, in particular from the collectionbasin or recirculation loop of these. Hence, the bulk of the particulatematerial in the gas stream is for example captured by scrubbing withscrubbing liquid comprising such high concentrations of dissolvedmaterial.

Introducing a liquid having a lower dissolved material concentration(e.g. urea concentration) than used in a first Venturi stage in the gasstream downstream of the first Venturi ejector may result in an increaseof the partial water vapor pressure in the gas stream. This may promotecondensation of water on submicron particles, resulting in an increaseof the particle size. This can improve capture of the now largerparticles in a downstream particle and/or droplet capture device, suchas for example (a part of) a Venturi ejector (e.g. a diverging tubepart) and/or mist eliminator. In particular condensation on submicronparticles and/or aerosol droplets containing urea in relatively higherconcentration, such as at least 50 wt. %, or even 100% urea, can bepromoted. In some embodiments, the method comprises evaporating at least0.001 kg/Nm³ or at least 0.005 kg/Nm³ or at least 0.010 kg/Nm³ waterdownstream of a first Venturi ejector stage and upstream of the throatof a Venturi ejector and/or droplet removal device.

Preferably, the gas stream is obtained from a urea finishing section,such as a urea granulator or a urea prilling tower, more preferably aurea prilling tower. The urea prilling tower is for example a forceddraft or a natural draft urea prilling tower.

The present method and system are especially advantageous for use withnatural draft urea prilling towers. Natural draft urea prilling towersare urea prilling towers which do not use a fan and/or blower to movethe cooling air through the prilling zone of the urea prilling tower.Typically such towers do not use fans or blowers to move cooling airthrough the urea prilling tower. Typically urea prilling towers are ofthe forced draft type (fans at bottom), induced draft type (fans at thetop), or natural draft type. A natural draft urea prilling tower maystill use an ejector e.g. in the off-gas treatment system.

Optionally, the process further comprises a step of urea prilling orurea granulation. Optionally, the process comprises solidification of aurea melt to produce urea prills or granules, using air for cooling ofthe urea melt droplets.

In some embodiments, the gas stream contains a concentration ofsubmicron particles greater than 20 mg/Nm³, or greater than 50 mg/Nm³,more preferably such concentration of urea particles. Submicronparticles have a particle size of 1.0 μm or less. Optionally, thepercentage of submicron particles is at least 0.5 wt. % and/or at most5.0 wt. % of the total weight of the particles in the gas stream,preferably of particles smaller than 1.0 μm, and optionally the amountof such particles is in the range 1.0 to 4.0 wt. %.

Preferably, the submicron particles are hydrophilic. Preferably, thesubmicron particles comprise a hygroscopic material. Preferably, thesubmicron particles are dissolvable in the scrubbing liquid, e.g. inwater. As used herein, submicron particles encompass for examplecolloidal aerosols. Condensation may optionally involve condensationonto a particle, droplet, or colloidal aerosol, causing it to grow insize.

The Venturi ejector as used herein is a type of Venturi scrubber andgenerally comprises, consecutively in series in the direction of the gasflow, a converging tube part, a throat, and a diverging tube part,wherein the converging and diverging part are usually conical tubeparts. The throat usually provides a narrow opening for passage of thegas stream and liquid supplied into the gas stream upstream of thethroat. The throat may be provided for example by a joint between twoparts, e.g. tube parts, or for example by a tube internal cross-sectionminimum. The acceleration and/or high velocity in the throat and/orconverging part contributes to intimate mixing of the gas and liquid,and to turbulence and atomization of the liquid. At least some particlesin the gas stream impact on droplets and are entrained and may beremoved in a downstream droplet removal device.

Preferably, the Venturi ejector (e.g. ejector Venturi scrubber)comprises a nozzle positioned for spraying scrubbing liquid in adirection parallel with the gas flow (of the gas stream to be treated)through the gas inlet of the Venturi ejector. Optionally, the centerlineof the nozzle is parallel to the gas flow. Preferably, the nozzle isinserted into a converging part, such as a conical tube part, of aVenturi ejector. In some embodiments, the nozzle is spaced apart from awall of a Venturi tube or duct part. Preferably, the gas inlet of theVenturi ejector is an opening extending substantially perpendicular(e.g. at an angle between 60° and 120° or between 85° and 95°) to theline from the nozzle to the throat of the Venturi. Preferably, the gasinlet of the Venturi ejector is arranged substantially parallel to theopening of the throat. Preferably, the gas flow centerline of the gas tobe treated does not bend between nozzle and throat (irrespective ofconverging of the flow). Preferably, the nozzle is positioned forspraying perpendicular to the throat cross-section and preferably thenozzle is centered to the throat cross section. Generally, the nozzle isspaced and upstream from the throat cross section. In some embodiments,liquid is supplied into a round throat Venturi ejector only through onesuch nozzle.

The nozzle used to introduce the scrubbing liquid in a Venturi ejectoris for instance hydraulic, producing small droplets through highpressure, or for example a dual fluid nozzle, wherein a liquid and anauxiliary gas stream, typically at pressure, flow both through thenozzle. Small droplets can be produced through shear forces between theliquid and gas that both travel through the nozzle.

The scrubbing liquid spray acts as motive fluid for the Venturi ejector,together with the air stream in case of a dual fluid nozzle. Hence, theVenturi ejector may acts as Venturi eductor wherein the gas stream to betreated is drawn in with the motive fluid stream. It may be noted thathigh energy Venturi scrubbers (with initial liquid velocity lower thanthe gas velocity) and ejector Venturi scrubbers (initial liquid velocityhigher than the gas velocity) have entirely different energyconsumption, atomization and scrubbing characteristics. The presentinvention involves ejector type Venturi scrubbers. The kinetic energy ofthe high velocity liquid (with or without co-injected gas flow) is usedto atomize the liquid and to pump the gas stream to be treated,generally through the scrubbing system and connecting ducts. The Venturiejector is generally used with a downstream droplet eliminator, e.g. agravity or inertial impact separator to remove scrubbing liquid from thegas stream. In particular a downstream mist eliminator can be used.

Optionally, a basic reagent is added, for example selected from thegroup consisting of: caustic, lime, limestone, hydrated lime, fly ash,magnesium oxide, soda ash, sodium bicarbonate, sodium carbonate, andmixtures thereof. This may be used for removal of acidic gases from thegas stream. Preferably, the reagent is added to a scrubbing liquidsprayed into the gas stream. Optionally the scrubbing liquid of aVenturi ejector comprises such reagent.

Preferably, an acidic reagent is added, more preferably selected fromthe group consisting of: acetic acid, boric acid, carbonic acid, citricacid, hydrochloric acid, hydrofluoric acid, nitric acid, oxalic acid,phosphoric acid, sulfuric acid, and mixtures thereof. This may be usedfor removal of basic gases from the gas stream, such as ammonia.Preferably, an acidic reagent is added in case of urea finishingoff-gas. Preferably, sulfuric acid or nitric acid is added. Optionally,the scrubbing liquid of a Venturi stage comprises such acidic reagent,for example in the first (most upstream) Venturi stage, or in thedownstream second Venturi stage.

Preferably, the acid or basic reagent is added to a scrubbing liquidsprayed into the gas stream, preferably downstream of a first Venturistage and preferably also downstream of a second Venturi stage.Optionally acid or basic reagent is comprised in a scrubbing liquid of aVenturi ejector, for instance of a first or second or optional thirdVenturi stage. A scrubbing solution from an acid scrubbing stepcomprising ammonium salt is for example supplied to a holding tankand/or outside battery limits, in particular if the acid reagent isintroduced into the gas stream downstream of the first Venturi stage.

Preferably, the method involves acid scrubbing and dust scrubbing ofurea prilling off gas, preferably carried out on top of a preferablynatural draft urea prilling tower, i.e. in an abatement system locatedat the top of a urea prilling tower.

In some embodiments, a stream comprising dissolved urea, such as ablowdown and/or purge stream, for example from the first Venturi stageand/or a quench stage wherein a scrubbing liquid with or without acidicor basic reagent is used, is supplied to a recycle vacuum evaporationsection, to give water vapor and a concentrated urea solution. Therecycle vacuum evaporation section is preferably separate from andadditional to the evaporation section of a urea plant. The concentratedsolution comprising urea is supplied to (a stream to) the urea finishing(e.g. granulation or prilling) and the urea is included in the solidurea product, e.g. granules or prills. The vapor is condensed and thecondensate is preferably reintroduced in the described method as make upwater, e.g. used for scrubbing the prilling off-gas with an aqueoussolution comprising less than 5 wt. % urea, such as in the secondVenturi stage. In case the process comprises acid scrubbing, the streamand concentrate may further comprise ammonium salts. A concentrate mayalso be supplied to a urea ammonium nitrate (UAN) plant or urea ammoniumsulfate (UAS) plant and introduced into a UAN or UAS product stream. Insome embodiments, the method comprises acidic scrubbing downstream of afirst Venturi stage and/or first quench or scrubbing stage and theacidic scrubbing solution purge stream is disposed of separately fromthe scrubbing liquid used upstream of said acidic scrubbing. Thescrubbing liquid used in a stage upstream of the acidic scrubbingcomprising urea is subjected to such evaporation.

Preferably, the static (absolute) pressure at the exit of the Venturiejector is nearly the same or slightly larger relative to the gasentrance of the Venturi ejector, e.g. at least 90% or at least 100% orat least 105% of the static pressure at the entrance. Preferably, themethod comprises urea prilling in a natural draft urea tower and Venturiscrubbing with said Venturi ejector on top of said prilling tower,wherein the static pressure at the exit of at least one Venturi ejectoris larger than at the entrance of the Venturi ejector. The preferredinitial velocity and ratio of the scrubbing liquid can contribute tosuch advantageous static pressure at such exit of the Venturi ejector.

Preferably, at least some or all Venturi ejectors are arrangedsubstantially horizontally, e.g. for substantially horizontal flowthrough the throat, for instance at an angle of less than 20° or lessthan 10° to horizontal. This allows for a compact design. Also possibleis for instance that at least some or all of the Venturi ejectors have avertical orientation. In that case, the Venturi ejectors are arrangedfor flow downward through the throat. This allows for a small pressuredrop.

Optionally, the method comprises quenching the gas stream upstream ofthe first Venturi ejector, for example by a temperature decrease of atleast 10° C. or at least 20° C. or to a gas temperature of less than 60°C., or of 50° C. or less, for instance by spraying an aqueous solutionand evaporation of at least some water. Optionally, the quenching spraysolution comprises at least 10 wt. %, or at least 20 wt. % or at least30 wt. % dissolved material, e.g. urea. Optionally, the quench spraysolution is at least in part obtained from recirculated Venturiscrubbing liquid, such as of a first Venturi stage. Optionally, thequench spray solution essentially consists of water. The quench solutionis for example sprayed as fine mist and/or in cross flow or co-currentflow.

Optionally, the gas is passed through a droplet removal device, such asa demister, between a first and second Venturi stage and/or after asecond Venturi stage. Optionally the method comprises further downstreamparticle capture and/or gas treatment steps.

In some embodiments, the method is carried out at the top of a prillingtower, more preferably on top of a urea prilling tower. Preferably, atleast the Venturi ejector is placed on or at the top of a prillingtower, in particular a urea prilling tower.

Optionally, the method comprises one or more scrubbing steps, comprisingscrubbing the gas stream with a scrubbing liquid, e.g. by spraying. Themethod optionally comprises passing the gas stream through a thirdVenturi stage, e.g. a third Venturi ejector or a (high energy) Venturiscrubber. A third Venturi stage may for instance be positioned betweenthe first and second Venturi ejector stage or downstream of the secondVenturi ejector stage. An optional third Venturi stage may for instanceoperate with a scrubbing liquid comprising an acid reagent. In someembodiments, the draw is essentially provided by the Venturi ejectors.In the method, submicron particles are removed from the gas stream.Generally, particles larger than 1 μm are also removed. Soluble gasessuch as ammonia may also be removed.

The present invention also pertains to a gas stream treatment methodcomprising subsequently:

A. providing a gas stream, preferably comprising or essentiallyconsisting off-gas from a urea finishing section, more preferably a ureaprilling tower, preferably from a natural draft urea prilling tower,

B. spraying an aqueous solution into the gas stream, preferablycomprising 20 wt. % to 55 wt. % urea, optionally to cool by at least 10°C. or at least 20° C. or to a temperature of less than 50° C.,preferably in cross-flow or co-current flow,

C. passing the gas stream through a first ejector Venturi scrubberhaving a throat, wherein an aqueous scrubbing liquid preferablycomprising 20 wt. % to 55 wt. % urea is sprayed into the gas stream inthe direction of the throat,

D. spraying an aqueous solution into the gas stream, preferably inco-current flow or cross-flow, preferably with a solution comprising 0wt. % to 5 wt. % urea, and optionally comprising an acid, for examplespraying with water,

E. optionally passing the gas stream through a mist eliminator,

F. passing the gas stream through a second ejector Venturi scrubberhaving a throat, wherein an aqueous scrubbing liquid preferablycomprising 0 wt. % to 5 wt. % urea is sprayed into the gas stream in thedirection of the throat,

G. optionally spraying an aqueous solution into the gas stream,optionally comprising 0 wt. % to 5 wt. % urea, and/or comprising acid orbase reagent, optionally in co-current flow, cross-flow orcounter-current flow, and

H. optionally passing the gas stream through particle removal device,such as a mist eliminator.

Usually, this preferred method comprises passing the gas stream throughat least one mist eliminator after at least one Venturi stage.Preferably, this method is carried out in a system on top of a prillingtower (e.g. natural draft prilling tower), wherein the system comprisesthe Venturi stages. Preferably in spraying step D, second Venturi stagestep F and/or step G, the solution and/or liquid comprises less than 2wt. % urea, e.g. from 0 to less than 2 wt. % urea, such as 0-1.50 wt. %urea, and optionally consists of water. Introducing in steps D and G,optionally also in F, a liquid having relatively low urea concentrationcan promote condensation of water on submicron urea particles, resultingin particle growth and better removal in a particle capture device suchas in step F and H. Preferably at least 5 g water/Nm³ is evaporated inthese steps D, F and/or G. Optionally, further steps are included, forexample a mist elimination step upstream of step D, e.g. between steps Cand D. Possibly one or more or all of said steps are carried outconsecutively.

In step E and H, the mist eliminator is independently for instance aknitted wire mist eliminator, a (plain wire) mesh mist eliminator,and/or a vane mist eliminator, e.g. a corrugated plate, in particular achevron shaped vane mist eliminator. A knitted wire mesh mist eliminatoris especially suitable for droplets with 3-20 μm size, and typicallyworks on interception. The separation efficiency for instance drops from90% at 3 μm, to less than 20% for droplets smaller than 1 μm. Typicalvane mist eliminators can remove 99% of particles of 10 μm and larger,especially at lower pressures. Vane mist eliminators are based oninertial impaction. Vane mist eliminators are more effective at highervelocities and greater droplets sizes than wire mesh mist eliminators.E.g. at higher gas velocities re-entrainment occurs for mesh misteliminators.

Preferably, in step E and/or H, a vane type mist eliminator is used. Forinstance, by the upstream venturi ejector and spraying (preferablyco-current spraying, especially for step E or both step E and H)upstream of the preferably vane type mist eliminator of step E, step H,or each of E and H, the particles grow to a droplet diameter allowingfor good removal in step E and H, such as above 10 μm for in particularstep H, The invention also pertains to a gas stream treatment methodcomprising these steps A to H, preferably with the described Venturiejector wherein the scrubbing liquid has an initial velocity of at least25 m/s and wherein the ratio of scrubbing liquid and gas flow is between0.0005 and 0.0015 (m³/h)/(m³/h). Step A optionally comprises a step ofurea prilling in a urea prilling tower, such as a forced draft, induceddraft, and preferably natural draft urea prilling tower, typicallycomprising supplying air into said prilling tower and spraying ureasolution (e.g. urea melt) from a distributor at the top of urea prillingtower, so as to solidify urea, to obtain solid urea-comprising prills,yielding an exhaust stream at the top part of said urea prilling toweras said gas stream of step A.

The invention generally also pertains to a gas stream treatment systemcomprising at least one Venturi ejector. The Venturi ejector maycomprise a Venturi scrubber and upstream thereof a nozzle directed tothe throat of the Venturi scrubber, further comprising a pump in fluidconnection with said nozzle for pressurizing at least liquid supplied tosaid nozzle. Venturi ejectors with round throats as well as withrectangular throats can for example be used.

The system preferably comprises two Venturi ejector stages in series,preferably as described. Each Venturi ejector stage typically comprisesa Venturi ejector comprising a converging part, a throat, and adiverging part, and a nozzle for spraying into said throat. Preferably,said Venturi stages are placed in series with each other with respect tothe gas stream, with optionally one or more intermediate steps betweenthem such as a spraying step and/or a mist elimination step. Preferably,the system is placed on top of a urea prilling tower, especially anatural draft urea prilling tower.

In a possible configuration, gas flows vertically down through theVenturi ejectors of the gas stream treatment sections, which ispreferred if equipment size is not a constraint. Preferably, a gasstream treatment section comprising two Venturi ejectors in series, hasa configuration wherein said Venturi ejectors are horizontally arranged,for a desirable small equipment size. Hence, a preferred gas streamtreatment system, preferably for a method according to the invention,comprises two Venturi stages in series, wherein each of said two Venturistages comprises a horizontally placed Venturi ejector comprising aconverging part, a throat, and a diverging part, and a nozzle forspraying into said throat, wherein said Venturi stages are placed aboveeach other. Spraying includes for example injecting a liquid jet whichbreaks up into a spray, such that a spray is provided into said throat.Preferably, the horizontally placed Venturi ejectors extend at least inpart into a (e.g. vertical) scrubbing column. Preferably the Venturiejectors extend at least in part, or entirely, under or above one of themist eliminators and/or above at least one of the basins (reservoirs)for collecting liquids of such a scrubbing column. Preferably the twoVenturi stages are connected by a scrubbing column. Preferably thesystem comprises two adjacent scrubbing columns integrated in a singlecasing, e.g. with a vertical wall dividing a casing (such as a vessel)into at least two scrubbing columns, wherein said scrubbing columns areconfigured for upward gas flow through such scrubbing column.

A preferred gas stream treatment system comprises two Venturi stages inseries, with a spray section in between, wherein the spray section ismore preferably for spraying a fine mist so as to provide forevaporation of sprayed liquid downstream a Venturi stage and upstream ofa Venturi stage. Optionally, the system comprises a compressor in fluidcommunication with a dual fluid nozzle for providing compressed air.Preferably, the gas stream treatment system comprises a mist eliminatorbetween the two Venturi stages, and preferably a second mist eliminatordownstream of the second Venturi stage. Optionally, these features arecombined with the mentioned system having horizontally placed Venturiejectors.

Preferably each Venturi stage comprises a scrubbing liquid recirculationloop, in particular comprising a pump for pressurizing and recirculatingscrubbing liquid to said nozzle. Preferably, the Venturi stages haveseparate recirculation loops. The separate recirculation loops allow fordifferent chemical compositions of the scrubbing liquids of each stage.A recirculation loop may comprise a fluid communication line from acollection basin or sump where scrubbing liquid is collected to one ormore spraying nozzles.

In a further preferred embodiment, which can be combined, the gas streamtreatment system comprises a horizontally placed ejector Venturiscrubber comprising an open-ended converging tube or duct part, athroat, and an open ended diverging part and a spray nozzle positionedinside said converging part for spraying into said throat, and whereinthe open end of said converging part is a gas inlet for the gas streamto be treated.

Preferably, the system comprises a sprayer between a first and adownstream second Venturi stage, for spraying an aqueous solution havinga higher water concentration (i.e. lower concentration dissolved andparticulate material) than the scrubbing liquid of the first Venturistage.

Optionally, the gas stream treatment system has a fluid connection witha urea finishing section, in particular for off-gas of a ureagranulation section of urea prilling tower. Optionally, the gas streamtreatment system is located at the top of a urea prilling tower, such ason top of the tower.

The invention also pertains to a urea prilling tower, preferably anatural draft urea prilling tower, having a gas stream treatment systemcomprising a Venturi ejector at the top of the prilling tower, morepreferably comprising two Venturi ejectors in series, and even morepreferably a gas stream treatment system as described. Preferably, thegas stream treatment system comprises a section configured for dustscrubbing and a downstream section for acid scrubbing, wherein bothsections are positioned at the top of a urea prilling tower. A gastreatment system at the top of a prilling tower is for example a gastreatment systems having an inlet at an elevation which is less than 5 mhigher or less than 5 m lower than the outlet for off-gas of theprilling tower.

Optionally, the gas stream treatment system does not comprise a fan orblower. Preferably, the system does not comprise a fan or blower forproducing a pressure drop of the gas stream.

Advantages of the method and system of the invention include a lowpressure drop, good efficiency for removal of submicron particles, andcompact design. Pressurizing a liquid such as the recirculatingscrubbing liquid can be efficiently done using compact equipment, e.g. apump. Atomizing the liquid with compressed air can also be efficientlydone using compact equipment, e.g., a compressor. Further advantageousperformance is achieved through the preferred inclusion of multiplestages employing progressively less concentrated aqueous solutions (e.g.lower urea concentration) to promote particulate growth by condensationon the surface of submicron particulate.

The invention also pertains to a method of modifying an existing plant,such as a urea finishing section and/or prilling tower, in particular aurea prilling tower, preferably a natural draft urea prilling tower, themethod comprising adding a gas stream treatment system, preferably withtwo Venturi ejector stages in series, preferably a gas stream treatmentsystem as described. The invention also pertains to a method ofmodifying an existing urea finishing section and/or prilling tower, inparticular an existing urea prilling tower, preferably a natural drafturea prilling tower, the method comprising adding a gas stream treatmentsystem comprising a Venturi ejector, preferably on top of the prillingtower. Preferably the gas stream treatment system comprises two Venturistages in series. Preferably the system is a system as described and/orfor the described methods. The invention also pertains to a method ofmodifying an existing urea finishing section having a Venturi ejector,comprising adding a Venturi ejector in series with said Venturi ejector.

Referring now to the drawings in general, the illustrations are for thepurpose of describing a preferred embodiment of the invention andillustrating preferred features of the systems and methods, and are notintended to limit the invention.

FIG. 1 shows a non-limiting embodiment of the invention. The vessel 1contains all of the components. The vessel 1 contains a first verticalcolumn A and a second vertical column B connected essentially byhorizontal venturi ejectors 3 and 6. The first column A has a gas inlet18. Provided is an inlet zone 2 for the gas stream (e.g. at 90% to 110%ambient pressure) with spray nozzles 10. Spray nozzles 10 spray incross-flow with the gas stream and downward. Sprays 10 may spray asolution obtained from reservoir 4. Spray nozzles 10 may provide forquenching of the gas stream. Spray nozzles 10 may provide forcondensation of water on submicron particles and/or for scrubbing.Sprayed liquid with captured and dissolved particles is collected inreservoir 4 under spray nozzles 10. Further provided is Venturi ejector3, e.g. a Venturi eductor 3. Venturi ejector 3 comprises a nozzle 11.The nozzle 11 sprays scrubbing liquid from the converging tube part intothe throat 15 of the venturi ejector. The gas inlet opening 17 of theconverging part of the ejector 3 is parallel to the opening of throat15, these openings are vertical. Ejector 3 has a horizontal orientation.Dashed line 16 indicates that the gas inlet 18 of the gas inlet zone 2,the gas inlet of the ejector 3, and the gas outlet of Venturi ejector 3have a centre on a common line, contributing to low pressure drop. Thenozzle 11 is positioned inside the converging tube part, downstream ofand spaced from the gas inlet 17 of the Venturi ejector 3. At the bottomof the gas inlet zone 2, a concentrated water reservoir 4 is provided.The reservoir 4 is e.g. for recirculating urea solution with arelatively high urea concentration. A purge stream can be withdrawn fromthe recirculating stream. The recirculation is to nozzles 10. Such areservoir 4B is also provided in the second column B, e.g. forrecirculating urea solution used as scrubbing liquid in nozzles 11. Fromthe stream obtained from the outlet of ejector 3, droplets are removed,e.g. using a mist eliminator 5, e.g. a vane type mist eliminator. Theliquid including captured urea is collected in reservoir 4B. The liquidin reservoir 4B, optionally together with liquid from reservoir 4A, e.g.if reservoir 4B and 4A are merged, is recirculated through a loop 19 tonozzles 11 of Venturi ejector 3 (with pressurizing) and optionally tospray nozzles 10. A purge stream is withdrawn from reservoir 4B anddisposed of. The mist eliminator 5 can also comprises a mesh. Upstreamof mist eliminator 5 spray nozzles 14 are provided.

These nozzles 14 spray optionally in co-current direction with the gasflow and optionally as fine mist, for instance with droplet size of lessthan 300 μm or less than 200 μm. They typically spray dilute water (e.g.low urea concentration), e.g. from reservoir 7. The spray nozzles 14upstream of the mist eliminator 5 for instance spray an aqueous solutioncomprising less than 5 wt. % urea, such as essentially water. Thecompartment between Venturi ejector 3, or said spray nozzles 14, andsecond Venturi ejector 6, or mist eliminator 5, preferably providessufficient residence time to allow for evaporation of e.g. at least 50wt. % of the sprayed water and preferably for condensation on submicronparticles.

Further provided is a second Venturi eductor 6 for spraying with dilutescrubbing liquid. The arrangement of the nozzle 12 is the same as foreductor 3, hence parallel to the gas flow. Also provided is a dilutewater reservoir 7. Reservoir 7 can be provided with a recirculation loopcomprising a pump to the spray nozzle 12 of eductor 6. Also provided isa mist eliminator 8, e.g. a vane type mist eliminator, for eliminatingdroplets e.g. as formed in the Venturi eductor. Mist eliminator 8 maycomprise a mesh, chevron, or combinations of each. Mist eliminator 8 isshown with horizontal arrangement. Any arrangement, including vertical,of a mist eliminator is also possible.

Further provided is a scrubber 13 upstream of said mist eliminator 8,e.g. the mesh, for spraying liquid counter-current into said gas flow.Cross-flow and co-current flow spraying are also possible for nozzle 13.The sprays can also be directed to the mist eliminator, for cleaning themist eliminator 8. Reservoir 7 collects droplets from eductor 6 and fromsaid scrubber 14. A purge stream from reservoir 7 can be disposed ofe.g. by supplying to reservoir 4, in view of the evaporation of liquidfrom reservoir 4 in for instance spray 2. Acid or basic reagent isoptionally used for scrubbing in the scrubber 13 downstream of Venturieductor 6. In such a case, the purge stream from reservoir 7 ispreferably not supplied to reservoir 4, but is disposed of separately.More preferably, scrubber 13 uses acidic scrubbing solution, such as(dilute) nitric acid or sulfuric acid, for acidic scrubbing to removeammonia. Also provided is an exit duct 9 for venting to the environment.

In operation, the particle-laden gases enter the inlet zone 2 where hotgases are initially cooled by evaporation of particle-concentrated water(e.g. an aqueous urea solution) from spray nozzles 10. The gases enterthe first Venturi eductor 3 where the liquid motive force from sprayingscrubbing liquid through the nozzle 11 propels the gases forward withoutrequiring a pressure drop. In the first Venturi eductor 3, the gases arescrubbed with the scrubbing liquid. The gas stream typically is orbecomes saturated with water, based on the partial water vapor pressureof the scrubbing liquid. Coarse particulate is collected and dissolvedinto the water and further concentrated in the concentrated waterreservoir 4. A gas stream including droplets leaves the first Venturieductor 3 and passes through a mist eliminator 5 including thespraying/scrubbing 14. The gas stream then enters the second Venturieductor 6. Liquid from spray nozzles 12 and mist eliminator 5 and fromthe outlet of Venturi ejector 3 is collected in reservoir 4 at thebottom of the scrubbing column B. The water in reservoir 4 comprisese.g. at least 40 wt. % dissolved material and e.g. at least 80 wt. % ofthe total removed particulate matter (including scrubbed in spray 2). Inthe second Venturi eductor 6, optionally at least some water evaporates,optionally saturating the gases with more water. In each Venturieductor, gas and liquid are mixed and typically particles are entrainedin liquid droplets, and the droplets are removed from the gas streamdownstream of the Venturi eductor. Gases continue through the secondmist eliminator 8, including the spraying 13, and then exit the scrubberat exit duct 9. Liquid obtained from mist eliminator 8 is collected inreservoir 7 and may be recirculated to Venturi eductor 6. The liquid inreservoir 7 comprises e.g. less than 5 wt. % urea and e.g. less than 20wt. % of total urea removed from the gas stream. The liquid 7 comprisesammonium salts if acidic scrubbing is used for nozzles 13, in such casethe purge stream from reservoir 7 is not supplied to reservoir 4, but isdisposed of separately.

The Venturi eductors (3, 6) are preferably mounted in a horizontalposition thereby allowing a compact design minimizing height, andpreferably above each other to reduce footprint.

The material of construction of the scrubber casing and the venturieductors is optionally a light-weight material such as FRP (FiberReinforced Plastic).

The pressure drop over the scrubber is typically less than 250 Pa, butcan be zero, depending on the inlet and outlet ducting pressure drop. Inmany instances, there will be a net pressure gain, for example of up to500 Pa or even more, with the outlet pressure being higher than in theinlet pressure.

FIG. 2 shows an embodiment wherein the system comprises a plurality ofparallel Venturi ejectors (3) and a second Venturi stage with aplurality of parallel Venturi ejectors (6). Herein the scrubbing columnand the venturi ejectors are integrated in a single casing (1). Venturiejector (3) extends horizontally under mist eliminator (5) and/orreservoir (7). The second Venturi ejector (6) stage extends horizontallyabove mist eliminator (5) and/or reservoir (7). A wall is providedbetween the two parts, at both sides in contact with process streams,separating the two halves, and only allowing for gas flow from one sideto the other side through the Venturi ejectors (3,6). The diverging part21 of the Venturi ejectors (3, 6) is provided at the upper side with anelbow 22 for gas/liquid separation having a bottom outlet 23 between thediverging part 21 end and a vertical separation wall 24.

FIG. 3 shows a typical particle size distribution and cumulative massfor urea dust in off-gas from a prilling tower.

FIG. 4 shows a typical particle size distribution and cumulative massfor urea dust in off-gas from a urea granulator.

Example 1

The invention will now be further illustrated by an example, which doesnot limit the invention. As an example of a preferred embodiment, aprilling tower is proposed that has a urea-laden airflow that needs tobe scrubbed. The temperature of the air leaving the prilling tower is80° C., the molar fraction of water vapor is 2%, and the concentrationof urea dust is 25 μg/g of gas flow. A Venturi eductor (ejector) sprayis provided which will cool the air by evaporation until the airflow issaturated and water no longer evaporates. Using thermodynamiccalculations in combination with steam tables, it is determined thatthis will occur at a final gas temperature of 33° C. with a water vapormolar fraction of 2.5% when using pure water. For this proposed projectthe amount of water evaporated is calculated to be 0.03 kg/Nm³ of gasflow. However, in practice, the Venturi eductor spray will be recycleduntil the urea concentration approaches 45% wt. At this ureaconcentration, the vapor pressure of water is proportionally less. UsingRaoult's law, the above calculations are repeated to find that the newsaturated gas temperature is 37° C. with a water vapor molar fraction of2.2%. Even though the saturated temperature is 4° C. higher, the molarfraction of water in the gas state is more than 10% less. Only 0.02 kgof water per Nm³ of gas flow is predicted to evaporate, for the proposedproject. Downstream of the concentrated Venturi eductor spray, when thegases are exposed to dilute water (e.g. aqueous urea solution having alower urea concentration, such as essentially no urea), the saturatedconditions will match the first case, enabling an additional 0.01 kg/Nm³of evaporation. By using a second dilute water spray that is preferablyat least 0.2 seconds, or preferably 0.3 seconds, upstream of the diluteVenturi eductor (second Venturi stage), submicron particulate growth ispromoted. This improves particle capture in for example a Venturieductor. The dilute water spray is for example implemented as spray in amist eliminator downstream of first Venturi stage and upstream of asecond Venturi stage, allowing for condensation on submicron particles.A quench spray upstream of a first Venturi stage may promote initialcondensation on submicron particles.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims and specification, the word “comprising”does not exclude other elements or steps, and the indefinite article “a”or “an” does not exclude a plurality. Terms such as “usually”,“generally”, “typically”, “in particular”, “can”, “may” and “suitably”indicate non-essential features which may be omitted in some embodimentsand can be combined with preferred features. The mere fact that certainfeatures of the invention are recited in mutually different dependentclaims does not indicate that a combination of these features cannot beused to advantage. Features of the methods can be combined with featuresof the system and features of the embodiments can be combined withfeatures illustrated in the drawings. The preferred methods can forexample be carried out in preferred systems and apparatuses.

The invention claimed is:
 1. A method for removing submicron particlesfrom a gas stream containing submicron particles said stream beingoff-gas from urea finishing, the method comprising: contacting the gasstream in a Venturi ejector with an injected high velocity scrubbingliquid to provide a pumping action, wherein the scrubbing liquid has aninitial velocity of at least 25 m/s and wherein the ratio of scrubbingliquid and gas flow is between 0.0005 and 0.0015 (m³/h)/(m³/h), wherebythe method comprises treatment in two Venturi stages in series, whereineach of said two Venturi stages comprises a horizontally placed Venturiejector, wherein said Venturi stages are placed above each other, andwhereby the two Venturi stages are connected by a scrubbing column. 2.The method of claim 1, wherein the gas stream is the off-gas from a ureagranulator.
 3. The method of claim 1 wherein the gas stream is theoff-gas from a urea prilling tower.
 4. The method of claim 3 wherein theresidence time between the first and the downstream second Venturiejector is at least 0.4 s.
 5. The method of claim 4 wherein theresidence time between the first and the downstream second Venturiejector is more than 0.8 s.
 6. The method of claim 1, wherein the gasstream contains a concentration of submicron particles greater than 20mg/Nm³.
 7. The method of claim 1 wherein the Venturi ejector comprises anozzle positioned for spraying scrubbing liquid in a direction parallelwith the gas flow through the gas inlet of the Venturi ejector.
 8. Themethod of claim 1 wherein a basic reagent is added selected from thegroup consisting of: caustic, lime, limestone, hydrated lime, fly ash,magnesium oxide, soda ash, sodium bicarbonate, sodium carbonate, andmixtures thereof.
 9. The method of claim 1 wherein an acidic reagent isadded selected from the group consisting of: acetic acid, boric acid,carbonic acid, citric acid, hydrochloric acid, hydrofluoric acid, nitricacid, oxalic acid, phosphoric acid, sulfuric acid, and mixtures thereof.10. The method of claim 9, wherein the method comprises acid scrubbingand dust scrubbing, both carried out at the top of a natural draft ureaprilling tower.
 11. The method of claim 1, further comprising the stepof urea prilling in a natural draft urea tower and wherein said Venturiejector is positioned at the top of said prilling tower, and wherein thestatic pressure at the exit of at least one Venturi ejector is largerthan at the entrance of the Venturi ejector.
 12. The method of claim 1,comprising subsequently: A) providing a gas stream, B) spraying anaqueous solution into the gas stream, C) passing the gas stream througha first ejector Venturi scrubber having a throat, wherein an aqueousscrubbing liquid is sprayed into the gas stream in the direction of thethroat, D) spraying an aqueous solution into the gas stream, and F)passing the gas stream through a second ejector Venturi scrubber havinga throat, wherein an aqueous scrubbing liquid is sprayed into the gasstream in the direction of the throat.
 13. A method for removingsubmicron particles from a gas stream, according to claim 12, comprisingsubsequently: providing a gas stream comprising off-gas from a ureaprilling tower, spraying an aqueous solution comprising 20 wt. % to 55wt. % urea into the gas stream, passing the gas stream through a firstejector Venturi scrubber having a throat, wherein an aqueous scrubbingliquid comprising 20 wt. % to 55 wt. % urea is sprayed into the gasstream in the direction of the throat, spraying an aqueous solutioncomprising 0 wt. % to 5 wt. % urea into and co-currently with the gasstream, passing the gas stream through a mist eliminator, passing thegas stream through a second ejector Venturi scrubber having a throat,wherein an aqueous scrubbing liquid comprising 0 wt. % to 5 wt. % ureais sprayed into the gas stream in the direction of the throat, sprayingan aqueous solution comprising 0 wt. % to 5 wt. % urea into the gasstream, and passing the gas stream through a mist eliminator.
 14. A gasstream treatment system, comprising two Venturi stages in series,wherein each of said Venturi stages comprises a horizontally placedVenturi ejector comprising a converging part, a throat, and a divergingtube part, and a nozzle for spraying into said throat, wherein saidVenturi stages are placed above each other, and wherein each Venturistage comprises a scrubbing liquid recirculation loop comprising a pumpfor pressurizing and recirculating scrubbing liquid to said nozzle,wherein the Venturi stages have separate recirculation loops, furthercomprising a sprayer between a first and a downstream second Venturistage, for spraying an aqueous solution having a higher waterconcentration than the scrubbing liquid of the first Venturi stage. 15.A urea prilling tower having a gas stream treatment system at the top ofthe prilling tower, said gas stream treatment system comprising twoVenturi stages in series, wherein each of said Venturi stages comprisesa horizontally placed Venturi ejector comprising a converging part, athroat, and a diverging tube part, and a nozzle for spraying into saidthroat, wherein said Venturi stages are placed above each other, andwherein each Venturi stage comprises a scrubbing liquid recirculationloop comprising a pump for pressurizing and recirculating scrubbingliquid to said nozzle, wherein the Venturi stages have separaterecirculation loops.
 16. A natural draft urea prilling tower, having agas stream treatment system at the top of the prilling tower, whereinthe gas stream treatment system comprises at least two Venturi ejectorscrubbers in series.
 17. The natural draft urea prilling tower of claim16, wherein the gas stream treatment section comprises two Venturistages in series, wherein each of said Venturi stages comprises ahorizontally placed Venturi ejector comprising a converging part, athroat, and a diverging tube part, and a nozzle for spraying into saidthroat, wherein said Venturi stages are placed above each other, andwherein each Venturi stage comprises a scrubbing liquid recirculationloop comprising a pump for pressurizing and recirculating scrubbingliquid to said nozzle, wherein the Venturi stages have separaterecirculation loops.
 18. A method of modifying an existing prillingtower, comprising installing a the gas stream treatment systemcomprising two Venturi stages in series, wherein each of said Venturistages comprises a horizontally placed Venturi ejector comprising aconverging part, a throat, and a diverging tube part, and a nozzle forspraying into said throat, wherein said Venturi stages are placed aboveeach other, and wherein each Venturi stage comprises a scrubbing liquidrecirculation loop comprising a pump for pressurizing and recirculatingscrubbing liquid to said nozzle, wherein the Venturi stages haveseparate recirculation loops on top of the prilling tower.
 19. Themethod of claim 18, wherein the existing prilling tower is a naturaldraft urea prilling tower.